Eel lifecycle

The European eel (Anguilla anguilla) is another species
that migrates between freshwater and the sea, but it adopts the
opposing (catadromous) life strategy to salmonids,
by breeding in a specific region of the North Atlantic (probably
the Sargasso Sea), from where ocean currents transport the young
eels (leptocephali) back to European coasts. The young eels
generally enter freshwater to grow and mature, although some may
remain in estuaries or coastal waters rather than moving upstream.
European eels are thought to comprise a single reproductive
population and all regional populations are therefore potential
contributors to the overall spawning escapement of the European
stock.

Diagram of the
life cycle of the European eel (redrawn from White and Knights,
1994)